Very common, widespread, rock-forming carbonate mineral, one of two polymorphs of CaCO3, the other being aragonite; sp. gr. 2.7; hardness 3; trigonal; usually colourless or white, but may be shades of yellow, grey, green, red, or even brown or black; white streak; vitreous lustre; crystals common, often tabular, prismatic, or rhombohedral, but fibrous aggregates and granular masses may also occur; cleavage perfect rhombohedral {101−1−}; cleavage rhombs exhibit double refraction; a major constituent of calcareous sedimentary rocks, e.g. marbles. Calcite can be precipitated from sea water and is a common constituent of invertebrate shells, and late-stage hydrothermal solutions (see hydrothermal activity) may precipitate calcite in cavities in some igneous rocks such as basalts. It is soluble in dilute hydrochloric acid. It is used as a flux, in cement-making and fertilizers, and as a building stone. See also carbonates.